Messages in this thread |  | | Date | Wed, 13 Nov 2013 16:50:20 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: nohz problem with idle time on old hardware |
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On Wed, 13 Nov 2013, Steven Rostedt wrote: > > I'm not saying that we are actually getting into nohz, but something > > with the nohz code is messing with cpu accounting. > > The trace does indeed show that a tick is happening, as the config has > HZ=250 (4ms) and we see a tick happen every 4ms. But for some reason, > we don't update the the idle time correctly when nohz is enabled. > > When I say nohz is enabled, I mean that we don't have nohz=off in the > command line. There seems to be some difference between having nohz=off > and having nohz disabled at runtime.
Right that affects tick_nohz_enabled
Two files use this variable: kernel/rcu/tree_plugin.h kernel/time/tick-sched.c
The only accounting related stuff is in tick-sched.c:
get_cpu_idle_time_us() and get_cpu_iowait_time_us()
Both functions bail out if (!tick_nohz_enabled).
The users of get_cpu_idle_time_us() are cpufreq and fs/proc/stat.c!
Now the simplest fix is to let those functions check whether we actually switched into NOHZ mode. Should work for the RCU tree stuff as well.
Thanks,
tglx
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